Recently, the projector market has grown along with widespread use of personal computers. Known display elements for modulating light beams with image information in a projector include small reflective elements, such as digital mirror devices (hereinafter termed the DMDs) from Texas Instruments, and transmission-type and reflection-type liquid crystal display elements arranged in a regular array. Reflection-type display elements are suitable for creating small pixels highly efficiently and have become the focus of attention as display elements for producing projected images of high quality.
Projection display devices using reflection-type liquid crystal display elements are disclosed, for example, in Japanese Laid-Open Patent Application H10-268235 and Japanese Laid-Open Patent Application 2001-154152. FIGS. 28 and 29 show known structures of projection display devices that use reflection-type liquid crystal display elements. The devices shown in FIGS. 28 and 29 separate white light from a light source (not shown) into three light beams of different colors, for example, green, blue, and red, and modulate the three light beams with image information by reflection from reflection-type liquid crystal display elements. As shown in FIG. 28 and FIG. 29, the light beams are modulated with image information by reflection from reflection-type liquid crystal panels 321a to 321c (FIG. 28) or 421a to 421c (FIG. 29) and then are combined before they reach a projection lens 330 (FIG. 28) or 430 (FIG. 29). The reflection-type liquid crystal panels 321a to 321c or 421a to 421c correspond to the three color light beams, for example, green, blue, and red in an arbitrary order, which is true in general of the prior art embodiments described herein. In FIG. 28 and FIG. 29, the optical paths of the respective color light beams are shown schematically. In FIG. 28, the solid and dotted lines indicate two different polarization states.
In the projection display devices of FIG. 28 and FIG. 29, the reflection-type liquid crystal panels 321a to 321c (FIG. 28) and 421a to 421c (FIG. 29) face the prism surfaces of polarization sensitive beam splitting prisms (hereinafter termed PBSs) 319 and 326 (FIG. 28) and 419a to 419c (FIG. 29) for separating an illumination light beam that comes from a light source (not shown in the drawings) from a projection light beam that travels to the projection lens via quarter-wavelength plates 320a to 320c (FIG. 28) or 420a to 420c (FIG. 29) for improving the contrast of a projected image. The device of FIG. 28 includes dichroic prisms 329 and 343, a wavelength specific polarization conversion element 342 for converting the polarization of a specific wavelength band of a light beam, and prism joints 318, 322, 325, and 327. The prism joints 325 and 327 include the wavelength specific polarization conversion elements. The device of FIG. 29 includes dichroic mirrors 416 and 470, a total reflection mirror 474, and an X-shaped dichroic prism.
Projection display devices that use transmission-type liquid crystal display elements are disclosed, for example, in Japanese Patent Publication 3175411 and Japanese Laid-Open Patent Application H09-61711. FIGS. 30 and 31 show known structures of projection display devices using transmission-type liquid crystal display elements. The devices shown in FIGS. 30 and 31 separate white light from a light source (not shown) into three light beams of different colors, for example, green, blue, and red, and modulate the three light beams with image information by transmission through the transmission-type liquid crystal display elements. As shown in FIG. 30 and FIG. 31, the light beams are modulated with image information by transmission through the transmission-type liquid crystal panels 571a to 571c (FIG. 30) or 671a to 671c (FIG. 31) and then are combined before they reach a projection lens 530 (FIG. 30) or 630 (FIG. 31). These devices include dichroic mirrors 516 and 570 (FIG. 30) and 616 and 670 (FIG. 31) for separating the light beams of different colors, condenser lenses 577a to 577c (FIG. 30) and 677a to 677c (FIG. 31), and total reflection mirrors 574a and 574b (FIG. 30) and 674a to 674c (FIG. 31). The device shown in FIG. 30 includes field lenses 580a to 580c on the projection lens side of the transmission-type liquid crystal panels 571a to 571c and dichroic mirrors 578 and 579 for combining the modulated light beams of different colors. In this device, the projected light beam modulated with image information is projected by one of the field lenses 580a to 580c and the projection lens 530. The device shown in FIG. 31 includes an X-shaped dichroic prism for the light beams of different colors that are modulated with image information.
As seen from the projection display devices described in Japanese Laid-Open Patent Applications H10-268235 and 2001-154152, cited above, and the structures shown in FIGS. 28 and 29, the projection display devices using multiple reflection-type liquid crystal display elements requires an optical combiner for combining light beams from the display elements, for example, the dichroic prism 329 (FIG. 28) or the X-shaped dichroic prism 448 (FIG. 29) and light separating structures for separating the illumination light beam from the projection light beam, for example, the PBSs 319 and 326 (FIG. 28) and PBSs 419a to 419c (FIG. 29). This is also true if multiple DMDs are used as the display elements. In the prior art, the optical combiner and the light separating structures are included between the side of the projection lens opposite the enlarging side, that is, on the reducing side, and the display elements.
Therefore, the projection lens in the prior art projection devices is required to have a very large back focal length. However, projection lenses in recent projection display devices tend to have small back focal lengths and a large field angle in order to project an image to a large screen with a small distance between the projection lens and the screen. Projection lenses for use in front projectors intended for use in very limited spaces such as homes or in rear projection TVs where overall size and thickness of the projectors must be kept small are demanded to have a large field angle. It is difficult to design a wide-angle lens having excellent optical performance for a projection display device having a large back focal length as described above. A certain quality of optical performance can be obtained by using expensive glass materials, but this leads to high cost of the lens. A large back focal length requires lens components of the projection lens to have relatively large diameters, particularly on the enlarging end of the lens, which tends to increase both size and cost of the projection lens.
The projection display device described in Japanese Patent Publication 3175411 and the structure shown in FIG. 30 use field lenses relatively near the display elements, as shown for example in FIG. 30, by field lenses 580a to 580c. This avoids increasing the back focal length of the projection lens. Thus, the projection lens and the entire projection display device can be made small and compact. When the field lenses 580a to 580c have very large focal lengths, the optical system for combining the different color light beams becomes larger, which leads to the same problem of excessive size discussed above. On the other hand, a certain distance should be maintained for combining light beams of different colors, for example, as shown in FIG. 30 by the light beams transmitted through the field lenses 580a and 580b being combined together with another color light beam via two dichroic mirrors 578 and 579 in order to form a projection light beam. Therefore, the focal lengths have to be within a very limited range. Such limiting of the powers of the field lenses makes it difficult to design a projection lens generally. That is, it is difficult to obtain a lens system that has excellent optical performance and can be produced at low cost.
The projection display device described in Japanese Laid-Open Patent Application H09-61711 and the device shown in FIG. 31 use an X-shaped dichroic prism, shown in FIG. 31 as X-shaped dichroic prism 648 provided on the side of the projection lens 630 opposite the enlarging side, that is, on the reducing side, for combining light beams of three different colors. This facilitates the projection lens having a relatively small back focal length, and, accordingly, enables designing a lens system having excellent optical performance and low cost. However, with this structure, lines crossing the projected image, related to structural characteristics of the X-shaped dichroic prism 648, may be visible in the projected image.